In recent years, membrane separations have been used extensively in commercial processes such as gas separation, liquid separation, wastewater treatment, etc. A membrane is a thin semipermeable barrier that is capable of separating components of a chemical solution or particles from a fluid as a function of their chemical and physical properties when a suitable driving force is applied across the membrane. Membranes can control species transfer rates from one region to the other. Microporous flat film membranes based on homopolymers for such processes can be produced via a melt process followed by a stretching step. The process used for producing this type of microporous films is well known (e.g., M. L. Druin, J. T. Loft, and S. G. Plovan, xe2x80x9cNovel open-celled microporous film,xe2x80x9d U.S. Pat. No. 3,801,404 (1974); H. S. Bierenbaum, R. B. Isaacson, M. D. Druin, and S. G. Plovan, xe2x80x9cMicroporous polymeric films,xe2x80x9d IandEC Prod. Res. Develop, 13, 2 (1974); H. S. Bierenbaum, L. R. Daley, D. Zimmerman, and I. L. Hay, xe2x80x9cProcess for preparing a thermoplastic microporous film involving a cold stretching step and multiple hot stretching steps,xe2x80x9d U.S. Pat. No. 3,843,761 (1974); and Brazinsky, W. M. Cooper, and A. S. Gould, xe2x80x9cProcess for preparing a microporous polymer film,xe2x80x9d U.S. Pat. No. 4,138,459 (1979)). However, to date, membranes having pore size as small as about 1 nm and ranging up to about 200 nm that can be used in severe chemical and high temperature environments have not been produced by a melt process involving multicomponent, multiphase systems.
It is toward the fabrication of microporous flat film membranes having a pore size as small as 1 nm and having significant chemical resistance and thermal stability based on immiscible polymer blends, made by a melt process and post-film-forming treatments that are environment-friendly and economically viable, that the present invention is directed.
The citation of any reference herein should not be deemed as an admission that such reference is available as prior art to the instant invention.
In its broadest aspect, the present invention extends to a microporous membrane which is prepared by a melt process from an immiscible blend of a major and minor component, optionally further including a compatibilizing block copolymer, and extruded into a precursor film, the precursor film subsequently stretched to provide a stable reticulated or interconnected network of microcracks or crazes throughout the membrane, with a resultant porosity and pore size. The major component of the immiscible blend is preferably a polymer or a copolymer. The major component may be a polyolefin polymer, such as by way of non-limiting examples, polypropylene, polyethylene, and poly(4-methyl-1-pentene). The minor component of the immiscible blend is a component immiscible with the major component and is preferably a polymer, such as an immiscible polyolefin, polystyrene, a polyester or a copolymer, and may be present at about 1 percent to about 40 percent by weight of the total. A compatibilizing block copolymer, selected to increase the compatibility between the immiscible major and minor polymers or copolymers, may optionally be present at about 0.5 percent to about 25 percent of the total. An example of a compatibilizing block copolymer for a blend of polypropylene with polystyrene is hydrogenated styrene isoprene/butadiene block copolymer (SEEPS). Optional additional components may be included in the blend, such as a monomer, oligomer or surfactant, to impart certain characteristics to the surfaces of the microcracks.
In a process for preparing the microporous membrane, melt blending is carried out to uniformly disperse the minor component within the major component, including the optional compatibilizing block copolymer, and a non-porous precursor film is formed therefrom, for example by extrusion. The nonporous precursor film may be about 50 to about 300 micrometers in thickness, and comprises a matrix of major polymer in which inclusions or domains of the minor component are uniformly dispersed. Following extrusion, the precursor film is uniaxially or biaxially stretched in at least two steps to a final dimension by about 100% to about 700% with respect to the original dimension. A first cold-stretching step is performed at a temperature from about 15xc2x0 C. to about 25xc2x0 C., increasing the dimension of the precursor film in the stretching direction about 20 percent to about 30 percent, and the film is held under tension thereafter. In a second, hot-stretching step, the cold-stretched film is further stretched at a temperature of about 5xc2x0 C. to about 15xc2x0 C. below the glass transition temperature of the minor component, to a total increase in dimension of 100 percent to about 700 percent of the original dimension of the precursor film. After post-film-forming treatment, the film may be about 10 micrometers to about 50 micrometers in thickness. The film produced by the foregoing method comprises a matrix of the major polymer with inclusions or domains of the minor polymer distributed uniformly therein, the major polymer further comprising a three-dimensional reticulated or interconnected network of uniformly distributed microcracks of relatively uniform dimensions. If an optional compatibilizing block copolymer is used, the block copolymer may be found coating the minor component inclusion particles. Other optional components present in the blend and present at the surfaces of the microcracks may impart characteristics to the microcracks such as hydrophilicity. The pore size of the film may be from about 1 to about 200 nanometers, and the porosity about 5 percent to about 30 percent or higher.
In a further aspect of the invention, the invention is drawn to a microporous membrane or film of about 10 to about 50 micrometers in thickness comprising a major component with inclusions or domains of a minor component distributed uniformly therein, the inclusions or domains of the minor component optionally coated by a compatibilizing block copolymer, the major component further comprising a three-dimensional reticulated or interconnected network of uniformly distributed reticulated or interconnected microcracks of relatively uniform dimensions. The pore size may be from about 1 to about 200 nanometers, and the porosity about 5 percent to about 30 percent or higher. The major component is preferably a polymer, such as a polyolefin, examples including but not limited to polypropylene, polyethylene or poly(4-methyl-1-pentene). The minor component is immiscible with the major component and is preferably a polymer, such as an immiscible polyolefin, polystyrene or a polyester, and may be present at about 1 percent to about 40 percent by weight of the total. The optional compatibilizing block copolymer may be present at about 0.5 percent to about 25 percent of the total.
In yet a further aspect, a process for the preparation of a microporous membrane of about 10 to about 50 micrometers in thickness comprising a major component with inclusions or domains of a minor component distributed uniformly therein, the inclusions or domains of the minor component optionally coated with a compatibilizing block copolymer, the major component further comprising a three-dimensional reticulated or interconnected network of uniformly distributed microcracks of uniform dimensions of about 1 to about 200 nanometers, the major component being preferably a polymer, such as a polyolefin, and the minor component immiscible with the major component being present at about 5 to about 25 percent by weight of the total and being preferably a polymer, such as an immiscible polyolefin, polystyrene or a polyester, the compatibilizing block copolymer if present is provided at about 0.5 percent to about 25 percent by weight of the total, the process comprising the steps of:
a) preparing an immiscible blend system comprising the minor component uniformly dispersed in the major component, optionally further containing a compatibilizing block copolymer;
b) forming a non-porous precursor film of about 50 micrometers to about 300 micrometers in thickness from the immiscible blend system; and
c) uniaxially or biaxially stretching the non-porous precursor film in at least two steps, the first step being a cold-stretching step in which the precursor film is stretched about 20 to about 30 percent at about 15xc2x0 C. to about 25xc2x0 C., and held under tension thereafter, and the second stretching step being a hot-stretching step at a temperature of about 10xc2x0 C. to about 15xc2x0 C. below the glass transition temperature of the minor component, the hot-stretching step increasing the dimension of the film to about 100% to about 700% of its original dimension.
In all of the foregoing aspects of the invention, the major component is preferably a polymer, for example a polyolefin, such as by way of non-limiting examples, polypropylene, polyethylene or poly(4-methyl-1-pentene). The minor component is immiscible with the major component and is preferably a polymer, such as another immiscible polyolefin, or polystyrene, polyester, and may be present at about 5 percent to about 25 percent by weight of the total. The major component and the minor component independently may be hydrophobic, hydrophilic, amorphous or semicrystalline. The blend of major component and minor component may be about 95:5 to about 75:25, however it is not so limiting. The extent of the increase in dimension of the precursor film to the final microporous membrane will depend on the selection of the components and their weight ratio, as well as the ratio of viscosity of the major to the minor component, the latter ratio controlling the domain size of the dispersed phase and hence the minimum attainable film thickness. It is also dependent on the optional presence of a compatibilizing block copolymer, of about 0.5 percent to about 25 percent by weight of the total, which allows for an increased stretching of up to about 600 percent to 700 percent of the original dimension of the precursor film. In addition, an optional further component may be present in the blend, such as a monomer, oligomer, or surfactant, one purpose of which is to provide certain desirable physicochemical characteristics to the surfaces of the microcrack network in the film. A non-limiting example of such additional components includes sodium dodecyl sulfate.
In a preferred embodiment of the foregoing aspects of the invention, the major component is polypropylene, and the minor component is polyethylene, polystyrene or a polyester such as poly(ethylene terephthalate). In more preferred embodiments, the ratio of polypropylene to polystyrene is 90:10 or the ratio or polypropylene to poly(ethylene terephthalate) is 85:15. In a blend containing a compatibilizing block copolymer, non-limiting examples include polypropylene:polystyrene 85:15 with about 7.5 percent by weight thereof block copolymer, and polypropylene:polystyrene 90:10 with about 5 percent thereof block copolymer. For this latter polymer blend, SEEPS is a preferred compatibilizing block copolymer.
The membranes or films preparable by the methods herein include not only flat films but other films or membranes of other shapes, such as but not limited to hollow membranes or fibers.
The minor component, when a polymer, may also be referred to as the first polymer component, and the major component, when a polymer, as the second polymer component.
It is thus an object of the present invention to provide a microporous membrane or film for use in separation processes, the membrane of film having domains of a first polymer component uniformly distributed in a matrix of a second polymer component, said second polymer component matrix comprising a three-dimensional network of uniformly distributed, interconnected microcracks of uniform dimension having a pore size of about 1 nanometer to about 200 nanometers and having a porosity of about 5 percent to about 40 percent, the membrane prepared from a film-forming composition, said film-forming composition consisting essentially of a mixture of
a first polymer component in an amount of from about 1 percent by weight to about 25 percent by weight, and
a second polymer component immiscible with said first polymer component and blended therewith, said second polymer component present in an amount ranging from about 65 percent by weight to about 99 percent by weight.
In one embodiment, the dimension of the microcracks is about 1 to about 10 nanometers or about 10 to about 20 nanometers. It can be as large as about 200 nm. The first and second polymer components may independently be amorphous, semicrystalline, hydrophilic or hydrophobic; they independently may be a polyolefin, such as polypropylene, polyethylene, or poly(4-methyl-1-pentene); polystyrene; or a polyester such as poly(ethylene terephthalate). In a non-limiting example, the first polymer component is 15 percent by weight poly(ethylene terephthalate) and the second polymer component is 85 percent by weight polypropylene; in another example, the first polymer component is 10 percent by weight polystyrene and the second polymer component is 90 percent by weight polypropylene.
In a further embodiment, the film may further include about 0.5 percent to about 25 percent by weight of a compatibilizing block copolymer. In a non-limiting example, a film may contain the compatibilizing block copolymer SEEPS, said first polymer is polystyrene and said second polymer is polypropylene; in one embodiment, a film is 10% by weight polystyrene, 90% by weight polypropylene, and about 5 percent by weight thereof SEEPS. In another embodiment, a film is 15% by weight polystyrene, 85% by weight polypropylene, and about 7.5 percent by weight thereof SEEPS. Another component, a monomer, an oligomer, a polymer, or a surfactant, may also be included in the mixture, with or without the compatibilizing block copolymer, to impart certain characteristics to the surfaces of the microcracks in the second or major component.
It is another object of the invention to provide for a method for the preparation of a microporous membrane useful in separation processes, the microporous membrane comprising domains of a first polymer component uniformly distributed in a matrix of a second polymer component, said second polymer component matrix comprising a three-dimensional network of uniformly distributed, interconnected microcracks of uniform dimension having a pore size of about 1 nanometer up to about 200 nanometers and having a porosity of about 5 percent to about 40 percent, said method comprising:
A. preparing a film-forming composition, said film forming composition consisting essentially of a mixture of
a first polymer component in an amount of from about 1 percent by weight to about 35 percent by weight,
a second polymer component immiscible with said first polymer component and blended therewith, said second polymer component present in an amount ranging from about 65 percent by weight to about 99 percent by weight;
B. preparing a film from the composition of step A; and
C. subjecting the film prepared in step B to a stretching procedure whereby said film is stretched at least 100% beyond the unstretched dimensions,
whereby the final microporous membrane is formed.
The film may be prepared, by way of non-limiting example, a casting, spray application to a substrate, extrusion, or any process for forming a film. After forming the film, the stretching procedure comprises a first cold stretching step followed by at least one hot stretching step. The cold stretching step is performed at a temperature of from about 15 C to about 25 C, and said film is thereby stretched to from about 20% to about 30% over its original dimension. The hot stretching step is performed at a temperature ranging from about 10 C to about 15 C below the glass transition temperature of the first polymer component, said hot stretching performed to the attainment of a final dimension ranging from about 100% to about 400% of the original dimension of the unstretched film.
In an optional further step, the film may be further treated by annealing under tension at a temperature of about 5 C. to about 10 C higher than the hot stretching step, but below the glass transition temperature of said first polymer component.
In another embodiment of the invention, the mixture further comprises a compatibilizing block copolymer, added to the film forming composition by the simultaneous mixture of the first and second polymer components and the compatibilizing block copolymer; or, the first polymer component and the compatibilizing block copolymer may be blended first, and subsequently mixed with the second polymer component. The formation of the film, and post-film-forming steps including stretching and optional post-stretching annealing may be in accordance with any of the examples as described above, but it is not so limiting.
It is yet a further object to provide a microporous membrane for use in separation processes, said microporous membrane comprising domains of a first polymer component uniformly distributed in a matrix of a second polymer component, said second polymer component matrix comprising a three-dimensional network of uniformly distributed, interconnected microcracks of uniform dimension with a pore size of about 1 nanometer to about 200 nanometers and a porosity of about 5 percent to about 40 percent, said microporous membrane prepared by the steps of:
A. preparing a film-forming composition, said film forming composition consisting essentially of a mixture of
a first polymer component in an amount of from about 1 percent by weight to about 35 percent by weight,
a second polymer component immiscible with said first polymer component and blended therewith, said second polymer component present in an amount ranging from about 65 percent by weight to about 99 percent by weight;
B. preparing a film from the composition of step A; and
C. subjecting the film prepared in step B to a stretching procedure whereby said film is stretched at least 100% beyond the unstretched dimensions,
whereby the final microporous membrane is formed.
The various means for forming the film, carrying out the stretching procedures, optional post-stretching annealing, are as described above. Furthermore, the a compatibilizing block copolymer may be included in the mixture, as described hereinabove, and the various processes inclusive thereof as exemplified above. An optional microcrack surface modifying component may also be included.
These and other aspects of the present invention will be better appreciated by reference to the following drawings and Detailed Description.